Conventional optical units are disclosed in Patent Literatures 1 and 2, respectively. The optical unit of Patent Literature 1 is mounted to a projector and includes a digital micromirror device, a first prism, a second prism, and a third prism. The digital micromirror device is a reflective image display element having an image display plane composed of a plurality of minute micro-mirrors.
The first prism has a reflecting surface that reflects illumination light, and the illumination light reflected from the reflecting surface is directed to the digital micromirror device. The digital micromirror device forms an image by illumination light intensity modulation in which a surface of each of the micro-mirrors is tilted by ON/OFF control. In the ON/OFF control, each of the micro-mirrors is driven uniaxially. A tilt angle of the micro-mirror in an ON state differs from a tilt angle of the micro-mirror in an OFF state.
The second prism has a total reflection surface that transmits ON light reflected from the ON-state micro-mirrors while totally reflecting OFF light reflected from the OFF-state micro-mirrors. The ON light transmitted by the total reflection surface is incident on the third prism. The third prism emits the ON light incident from the second prism toward an image projection side. In this way, the image is projected. Here, an optical axis of the ON light reflected from the digital micromirror device is disposed on a center plane thicknesswise of the first prism, the second prism, and the third prism. An optical axis of the OFF light reflected from the digital micromirror device is disposed on a center plane thicknesswise of the first prism and the second prism.
When incident on a projection lens, the OFF light (unwanted light) that is not used for a projection image causes ghost light. With the OFF light incident on the projection lens, a local temperature rise is caused to the projection lens, thereby degrading imaging performance of the projection lens. The projection image thus has lower contrast. According to the optical unit of Patent Literature 1, the OFF light reflects off the total reflection surface of the second prism for emission in a direction different from that of the ON light. The OFF light is thus prevented from being emitted toward the image projection side, whereby a projection image can be prevented from having lower contrast that might otherwise be caused by the OFF light.
Non-Patent Literature 1 discloses a digital micromirror device (Tilt & Roll Pixel DMD) in which micro-mirrors are each driven with respect to two orthogonal axes. In this digital micromirror device, the micro-mirrors reflect OFF light so that an optical axis of the OFF light is directed away from a plane including an optical axis of ON light and an optical axis of illumination light. By being mounted with the biaxial-control digital micromirror device, the optical unit of Patent Literature 1 or 2 can thus improve brightness of its projection image.